1. Field of the Invention
The invention relates to engine starting control of a hybrid vehicle.
2. Description of Related Art
In a hybrid vehicle driven by an engine and a motor, control for intermittently starting and stopping the engine during running of the vehicle is performed.
If the engine is intermittently stopped, the engine may be brought into an inappropriate or undesirable combustion state when the engine is started after being stopped, due to fuel deposited and remaining on an intake system of the engine when the engine is stopped. Therefore, it has been proposed to count the number of times the engine is intermittently stopped during running, and reduce the fuel injection amount when the engine is started next time, as the number of times of intermittent stopping increases, so as to provide an appropriate combustion state during starting of the engine (see, for example, Japanese Patent Application Publication No. 2011-235769 (JP 2011-235769 A)).
In a hybrid vehicle in which the engine is started when the vehicle required power exceeds a starting determination value, and the fuel supply amount is feedback-controlled based on a signal of an exhaust oxygen sensor or air-fuel ratio sensor, it has been proposed to reduce the starting determination value and start the engine earlier than usual, when a feedback amount determined from the signal of the exhaust oxygen sensor or air-fuel ratio sensor is equal to or larger than a predetermined value, so as to find a failure of the engine early (see, for example, Japanese Patent Application Publication No. 2010-052610 (JP 2010-052610 A)).
As described in JP 2010-052610A, the air-fuel ratio of exhaust gas is detected by the air-fuel ratio sensor, or the like, mounted in an exhaust system of the engine, and a signal indicative of the air-fuel ratio is fed back for control of the fuel injection amount, flow rate of air, etc. so that an air-fuel mixture is burned at the stoichiometric air-fuel ratio. Since exhaust gas containing a large amount of air is emitted immediately after engine starting, the air-fuel ratio sensor outputs a (lean state) signal indicating a large air-fuel ratio. Then, after the air-fuel ratio is once brought into a state (rich state) smaller than the stoichiometric air-fuel ratio, through the feedback control using the signal from the air-fuel ratio sensor, the air-fuel ratio gradually increases toward the stoichiometric air-fuel ratio, and is finally controlled to the stoichiometric air-fuel ratio. Therefore, it takes a certain period of time from the lean state immediately after engine starting to the time when the air-fuel ratio becomes equal to the stoichiometric air-fuel ratio.
In the meantime, since the engine is stopped and started according to the vehicle required power in the hybrid vehicle, the engine is started and stopped irrespective of the time required to stabilize the air-fuel ratio, if the vehicle required power rapidly changes, such as when the driver stops depressing the accelerator pedal immediately after the driver largely depresses the accelerator to start the engine. As a result, exhaust gas in a condition (lean state) where the air-fuel ratio is larger than the stoichiometric air-fuel ratio, or exhaust gas in a condition (rich state) where the air-fuel ratio is smaller than the stoichiometric air-fuel ratio, remains in the exhaust system after the engine is stopped. The remaining lean-state or rich-state exhaust gas may deteriorate the performance of a catalyst installed in the exhaust system for cleaning up exhaust gas, and the exhaust gas may not be appropriately cleaned up. Also, if the engine is started next time in a condition where exhaust gas whose air-fuel ratio deviates from the stoichiometric air-fuel ratio remains in the exhaust system after the engine is stopped, a period of time for which the air-fuel ratio of exhaust gas keeps deviating from the stoichiometric air-fuel ratio, namely, a period of time it takes the air-fuel ratio of exhaust gas to be stabilized to the stoichiometric air-fuel ratio is prolonged, and, consequently, the fuel efficiency of the engine may be reduced.